See also

Molybdenum (, from the Greek
meaning "lead-like"), is a
Group 6chemical
element with the symbol Mo and atomic
number 42. It has the sixth-highest melting point of any
element, and for this reason it is often used in high-strength
steel alloys. Molybdenum is found in trace amounts in plants and
animals, although excess molybdenum can be toxic in some animals.
Molybdenum was discovered in 1778 by Carl
Wilhelm Scheele and first isolated in 1781 by Peter
Jacob Hjelm.

Characteristics

Molybdenum is a transition
metal with an electronegativity
of 1.8 on the Pauling scale and an atomic mass of
95.9 g/mole. It does not react with oxygen or water at
room temperature. At elevated temperatures, molybdenum trioxide is
formed in the reaction 2Mo + 3O2 → 2MoO3.

In its pure metal form, molybdenum is silvery
white with a Mohs
hardness of 5.5, though it is somewhat more ductile than tungsten. It has a
melting
point of 2623°C, and only tantalum, osmium, rhenium, and tungsten have higher melting
points. It also has the lowest heating expansion of any
commercially used metal.

Isotopes

There are 35 known isotopes of molybdenum ranging
in atomic
mass from 83 to 117, as well as four metastable nuclear
isomers. Seven isotopes occur naturally, with atomic masses of
92, 94, 95, 96, 97, 98, and 100. Of these naturally occurring
isotopes, five are stable, with atomic masses from 94 to 98. All
unstable isotopes of molybdenum decay into isotopes of niobium, technetium, and ruthenium.

Molybdenum-92 and molybdenum-100 are the only
naturally occurring isotopes that are not stable. Molybdenum-100
has a half-life of
approximately 1×1019 y and undergoes double
beta decay into ruthenium-100. Molybdenum-98
is the most common isotope, comprising 24.14% of all molybdenum.
Molybdenum isotopes with mass numbers from 111 to 117 all have
half-lives of approximately .15 μs.

Though molybdenum is found in such minerals as wulfenite (PbMoO4) and powellite (CaMoO4), the main commercial source
of molybdenum is molybdenite (MoS2). Molybdenum is mined as a
principal ore, and is also recovered as a byproduct of copper and
tungsten mining.

A side product of molybdenum mining is rhenium. As it is always present
in small varying quantities in molybdenite, the only commercial
source for rhenium is molybdenum mines.

Compounds

Molybdenum has several common oxidation
states, +2 +3 +4 +5 and +6. The highest oxidation state is
common in the molybdenum(VI)
oxide MoO3 while the normal sulfur compound is molybdenum
disulfide MoS2. The broad range of oxidation states shows up in
the chlorides of molybdenum:

Biological role

The most important use of the molybdenum
atom in living organisms is as a metal hetero-atom at the active
site in certain enzymes. In nitrogen
fixation in certain bacteria, the nitrogenase enzyme which is
involved in the terminal step of reducing molecular nitrogen,
usually contains molybdenum in the active site (though replacement
of Mo with iron or vanadium is known).

Though molybdenum forms compounds with various
organic
molecules, including carbohydrates and amino acids,
it is transported throughout the human body as MoO42-. Molybdenum
is present in approximately 20 enzymes in animals, including
aldehyde
oxidase, sulfite
oxidase, xanthine
oxidase. It occurs in higher concentrations in the liver and
kidneys, and in lower concentrations in the vertebrae. Pork, lamb,
and beef liver each have approximately 1.5 parts molybdenum per
million. Other significant dietary sources include green beans,
eggs, sunflower seeds, wheat flour, lentils, and cereal grain.
Molybdenum deficiency is not usually seen in healthy people.
Sodium
tungstate is a competitive
inhibitor of molybdenum. Dietary tungsten reduces the
concentration of molybdenum in tissues. The condition can be
aggravated by excess sulfur. Most high-strength steel
alloys are .25% to 8% molybdenum.

Molybdenum 99 is used as a parent radioisotope to
the radioisotope Technetium 99, which is used in many medical
procedures

Molybdenum
disulfide (MoS2) is used as a lubricant and an agent. It forms
strong films on metallic surfaces, and is highly resistant to both
extreme temperatures and high pressure, and for this reason, it is
a common additive to engine motor oil; in case of a catastrophic
failure, the thin layer of molybdenum prevents metal-on-metal
contact. Lead molybdate co-precipitated with lead chromate and lead
sulfate is a bright-orange pigment used with ceramics and plastics.
Molybdenum
trioxide (MoO3) is used as an adhesive between enamels and metals. the principal
ore from which molybdenum is now extracted, was previously known as
molybdena. Molybdena was confused with and often implemented as
though it were graphite. Even when the two
ores were distinguishable, molybdena was thought to be a lead ore.

It was not until 1778 that Swedish chemist
Carl
Wilhelm Scheele realized molybdena was neither graphite nor
lead. He and other chemists then correctly assumed that it was the
ore of a distinct new element, named molybdenum for the mineral in
which it was discovered. Peter
Jacob Hjelm successfully isolated molybdenum using carbon and linseed oil
in 1781. For a long time there was no industrial use for
molybdenum. The French Schneider
Electrics company produced the first steel molybdenum alloy
armor plates in 1894. Until World War I
most other armor factories also used molybdenum alloys. In World War
I, some British tanks were protected by 75 mm manganese plating, but this
proved to be ineffective. The manganese plates were then replaced
with 25 mm molybdenum plating. These allowed for higher
speed, greater maneuverability, and, despite being thinner, better
protection.
OSHA regulations specify the maximum permissible molybdenum
exposure in an 8-hour day to be 5 mg/m³. Chronic exposure to 60 to
600 mg Mo/m³ can cause symptoms including fatigue, headaches, and
joint pains.

Supply and demand

Although current molybdenum production
meets demand, refiners, or roasters, are expected to run into a
shortfall between 2009 and 2015, depending on demand.

A roaster processes the moly into a fine powder,
pellets, or other forms. Total world moly roaster capacity is
currently 320 million pounds per year, barely enough to meet
demand. There is not much excess roasting capacity, and no one is
actively permitting for the production of any new roasters in the
United States. Global roaster capacity also looks limited, and a
future roaster shortage is predicted. The data above are based on
the assumption that mines will be able to increase output.

Western demand is projected to increase by around
3 percent annually, while China and the CIS demand is projected to
increase by around 10 percent annually, increasing overall global
demand by around 4.5 percent annually. Increasing demand can be
attributed to two main factors. Hydroprocessing catalysts are
becoming essential for crude oil. The other contributing factor is
the increase in nuclear reactor construction. There are 48 nuclear
reactors to be built by 2013, and approximately 100 are to be built
by 2020. The International Molybdenum Association (IMOA) says that
an average reactor contains about 520,000 feet of stainless steel
alloy. Some larger reactors contain over 1 million feet of
stainless steel alloy. Unless moly mine production picks up at a
rapid pace, shortfalls of the metal are expected to arrive around
2009.